JPH04368191A - Error signal generator in wavelength stabilization of laser light - Google Patents

Abstract

PURPOSE:To enable arbitrary setting of standard wavelength when wavelength stabilization is performed for laser light and to acquire an error signal which expresses a difference between the standard wavelength and a current laser oscillation wavelength by using interaction between laser light and wavelength conversion element. CONSTITUTION:A fully small a.c. element is added to a dc element of inrush current of a semiconductor laser 1 for amplitude modulation of inrush current. Thereby, extremely fine wavelength modulation is applied to laser light. When the laser light is injected to a wavelength conversion element 2 and a detection signal of laser light of angular frequency of 2omega whose wavelength is converted is synchronously detected by a lock-in amplifier 5, differential signal which is zero cross signal can be acquired.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to a difference between a target value and a current value, that is, an error signal generating device used in stabilizing the wavelength of a laser beam, and in particular, to a device that generates an error signal using the interaction between a laser beam and a wavelength conversion element. The present invention relates to an error signal generation device that obtains an error signal.

0002

[Prior Art] Stabilizing the wavelength of laser light is an indispensable technology for improving accuracy in optical measurement and increasing capacity and speed in optical communications. A method of stabilizing the oscillation wavelength to the reference wavelength is to extract the difference between the oscillation wavelength and the reference wavelength as an error signal, and to control the oscillation wavelength of the laser beam so that this error signal becomes zero.

Conventionally, the absorption line of atoms or molecules or the resonance wavelength of an optical resonator has been used as a reference wavelength to generate an error signal. A method using absorption lines of atoms and molecules will be explained below. FIG. 7a shows an absorption line of rubidium (one of the Rb-D2 lines (wavelength 780.0 nm)) often used for wavelength stabilization of semiconductor lasers. FIG. 7b shows a differential signal obtained by differentiating the amount of transmitted light of the absorption line with respect to wavelength, and this is used as an error signal.

FIG. 6 is a block diagram of the optical system and signal processing for wavelength stabilization of this semiconductor laser. In FIG. 6, 1 is a semiconductor laser, 20 is an Rb cell, and gaseous R
b is enclosed, 3 is a photoelectric converter, 4 is an oscillator, 5
is a lock-in amplifier, 6 is a controller, and 7 is an adder. The oscillation wavelength of the semiconductor laser 1 can be controlled by controlling the current injected into the semiconductor laser 1. Therefore, by injecting a laser beam into the Rb cell 20 and sweeping the injection current while measuring the transmitted light beam intensity with the photoelectric converter 3, the Rb absorption line shown in FIG. 7a can be observed.

Next, the generation of an error signal in stabilizing the wavelength of laser light will be explained. By superimposing a part of the output of the oscillator 4 on the output of the controller 6 using the adder 7, an alternating current component that is sufficiently smaller than the direct current component of the injected current is added to modulate the amplitude of the injected current. A very small wavelength modulation is applied to the oscillation wavelength of the laser beam. When the output of the photoelectric converter 3 is synchronously detected at the modulation frequency that is the output of the oscillator 4 using the lock-in amplifier 5 while sweeping the injection current in a state where wavelength modulation is performed, the Rb absorption shown in FIG. 7b is obtained. A line differential signal is obtained. This differential signal is a so-called zero cross signal, and serves as an error signal for wavelength stabilization.

The controller 6 controls the injection current of the semiconductor laser 1 so that this error signal becomes zero, thereby performing feedback control for wavelength stabilization. In this way, the oscillation wavelength of the semiconductor laser 1 is set at the zero cross point (wavelength = 780
nm). In the above example, when the modulation frequency is 5 kHz and the time constant of the lock-in amplifier is 100 msec, a wavelength stability of 10-13 is obtained.

[0007]

[Problem to be Solved by the Invention] However, when wavelength stabilization is performed using the absorption lines of atoms and molecules, the number of absorption lines of atoms and molecules is finite and discrete, so the wavelength to be stabilized is is limited to the wavelength of the absorption line, making it difficult to stabilize the wavelength of laser light for any wavelength.

Furthermore, when the resonant wavelength of an optical resonator is used as a reference wavelength, there are problems in that it is difficult to adjust the resonator and the mechanism for changing the reference wavelength becomes complicated. The present invention makes it possible to generate an error signal for wavelength stabilization of laser light at any wavelength by utilizing interaction between laser light and a wavelength conversion element.
An object of the present invention is to provide an error signal generation device for stabilizing the wavelength of laser light.

[0009]

[Means for Solving the Problems] In order to solve the above problems, the present invention includes means for modulating the wavelength of laser light, a wavelength conversion element, and a photoelectric converter for detecting the wavelength-converted light beam. It is characterized by

The present invention is also characterized in that it comprises a wavelength conversion element, means for modulating the interaction length between the laser beam and the wavelength conversion element, and a photoelectric converter for detecting the wavelength-converted light beam. .

The present invention also provides a beam splitter for splitting a laser beam into two light beams, a wavelength conversion element, two photoelectric converters for detecting each of the wavelength-converted light beams, and a photoelectric converter for detecting each wavelength-converted light beam. It is characterized in that it includes a subtracter that obtains a difference signal of each output, and is arranged so that the interaction lengths between the two light beams split by the beam splitter and the wavelength conversion element are different from each other.

0012

[Operation] The principle of the error signal generating device for stabilizing the wavelength of laser light according to the present invention will be explained below. The laser light
LiNbO3, KH2PO4, KTiOPO4
, KNbO3 or the like, a wavelength-converted laser beam is generated. Note that in order to perform wavelength conversion, a state in which phase matching is not achieved, that is, a condition of Δk≠0 in the example shown in (Equation 1) is required.

The intensity of the wavelength-converted laser light oscillates sinusoidally at a constant period by changing the interaction length between the incident laser light and the wavelength conversion element. This period is commonly referred to as the "coherence length." The coherence length changes depending on the wavelength dispersion of the refractive index of the wavelength conversion element and the wavelength of the incident laser light. vice versa,
Even if the interaction length is kept constant and the wavelength of the incident laser light is changed, the intensity of the wavelength-converted laser light changes sinusoidally.

[0014] In order to understand these phenomena, the following equation (1) is used to express the second harmonic generation as an example. Note that for ease of understanding, the angular frequency ω is used instead of the wavelength λ (ω=2π·c/λ).

[0015]

[Math 1]

Note that when 2ω and ω are far from the resonant frequency of the wavelength conversion element, the difference (n2
ω−nω) can be considered constant.

FIG. 1 is a graph of (Equation 1) in which the power of light 2ω is plotted on the vertical axis and the angular frequency ω is plotted on the horizontal axis. In Fig. 1, with the interaction length l as a parameter, the dashed line 1 indicates l=la, and the solid line 2 indicates l=lb (la<lb
) is the graph for the case. The sinusoidal signal in FIG. 1 serves as a reference for wavelength stabilization, and this differential signal serves as an error signal in wavelength stabilization of laser light. Further, by changing the interaction length l, the wavelength reference can be arbitrarily set.

Therefore, according to the configuration of the present invention, the angular frequency ω of the laser beam is modulated by making the laser beam incident on the wavelength conversion element and modulating the wavelength of the laser beam. The differential signal of the graph of is obtained. This differential signal is a sinusoidal signal without a DC component, and has a plurality of zero-crossing points. Therefore, by adjusting the interaction length l, a zero-crossing point with a slope that results in negative feedback is selected, and the angular frequency ω at that point is adopted as the reference angular frequency.The reference wavelength is determined and the laser beam is The error signal in wavelength stabilization can be obtained.

Furthermore, by changing the interaction length between the incident laser beam and the wavelength conversion element without modulating the wavelength of the laser beam, the intensity of the wavelength-converted laser beam can be made sinusoidal with a constant period. A vibrating phenomenon appears. Figure 2 shows
This is a graph of (Equation 1) in which the power of light 2ω is plotted on the vertical axis and the interaction length l is plotted on the horizontal axis. Note that λ is the wavelength of the incident laser beam.

Therefore, by modulating the interaction length between the laser beam and the wavelength conversion element, the differential signal shown in the graph of FIG. By selecting a zero-crossing point having an inclination of , and using the interaction length l at that point as a reference value, a reference wavelength can be determined, and an error signal in wavelength stabilization of laser light can be obtained.

In addition, the laser beam is divided into two light beams which are incident on the wavelength conversion element, and each light beam is arranged so that the interaction length with the wavelength conversion element is different. By generating a difference signal of the respective outputs from two photoelectric converters that detect the light beam, the dashed line 1 shown in FIG.
A difference signal between the graph of 2 and the graph of solid line 2 is obtained. This difference signal has a plurality of zero crossing points at the intersection of the broken line 1 and the solid line 2. Therefore, by adjusting the interaction length l,
A reference wavelength is determined by selecting a zero-crossing point with a slope that results in negative feedback and adopting the angular frequency ω at that point as a reference angular frequency, thereby obtaining an error signal in wavelength stabilization of laser light. Can be done.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of an error signal generation device for wavelength stabilization of laser light according to the present invention will be described. (Embodiment 1) FIG. 3 is an optical system and signal processing block diagram of an embodiment of an error signal generating device according to the present invention. Figure 3
, 1 is a semiconductor laser, and 2 is a wavelength conversion element, which is formed into a wedge shape as shown in the figure, and the interaction length l can be changed by moving it in the X direction. 3 is a photoelectric converter, 4 is an oscillator, 5 is a lock-in amplifier, 6 is a controller, 7 is an adder, 8 is a filter that transmits light with an angular frequency of 2ω and blocks light with an angular frequency ω, and 9 is a wavelength converter. This is an X-axis stage that moves the element 2 in the X direction.

By adding a sufficiently small alternating current component to the direct current component of the current injected into the semiconductor laser and modulating the amplitude of the injected current, extremely small wavelength modulation is applied to the laser light. When this laser light is incident on the wavelength conversion element 2 and the detection signal of the wavelength-converted laser light with an angular frequency of 2ω is synchronously detected by the lock-in amplifier 5, a differential signal obtained by differentiating, for example, the solid line graph in FIG. 1 is obtained. . This differential signal is a so-called zero cross signal and serves as an error signal for control.

The controller 6 can stabilize the wavelength of the laser beam by inputting this error signal and controlling the injection current of the semiconductor laser 1 so that the output of the lock-in amplifier 5 becomes zero. . Further, by moving the wavelength conversion element 2 in the X direction on the X-axis stage 9 and adjusting the interaction length l, the wavelength of the laser light to be stabilized can be arbitrarily set. The modulation frequency is set within the usable range of the lock-in amplifier, from several Hz to approximately 5 Hz.
A range of 0 MHz is preferred.

(Embodiment 2) FIG. 4 is an optical system and signal processing block diagram of another embodiment of the error signal generating device according to the present invention. In FIG. 4, 10 is a vibrator such as a piezo element that can vibrate the wavelength conversion element 2 in the X direction with extremely small amplitude. Since the wavelength conversion element 2 vibrates in the X direction with extremely small amplitude, the interaction length l is modulated. This corresponds to, for example, the solid line graph in FIG. 1 vibrating in the angular frequency ω direction. By synchronously detecting this signal with lock-in amplifier 5,
A control error signal, which is a zero-crossing signal similar to that obtained in the first embodiment, can be obtained.

The controller 6 can stabilize the wavelength of the laser beam by inputting this error signal and controlling the injection current of the semiconductor laser 1 so that the output of the lock-in amplifier 5 becomes zero. .

As a specific example, if a trapezoidal cross section with a base of 2 mm, a top of 1.96 mm, and a height of 20 mm is used as the wavelength conversion element 2, and it is vibrated with an amplitude of 0.1 mm using a piezo element. , the amount of displacement of interaction length l is 0.2μ
m. This corresponds to amplitude modulation of the light frequency of about 3.6×10 14 Hz at a frequency of about 36 GHz when the oscillation wavelength of the laser light is 830 nm. Further, the modulation frequency depends on the driving force of the piezo element, the mass of the wavelength conversion element, etc., but is preferably about 5 kHz to 6 kHz.

In this embodiment, unlike the first embodiment, there is no need to apply wavelength modulation to the laser beam when generating an error signal. When wavelength modulation is applied to a laser beam, wavelength fluctuations in the modulated frequency component remain in the wavelength-stabilized laser beam, so when used for measurement, communication, etc., accuracy, speed, capacity, etc. From this point of view, there is a tendency to limit the scope of use. Therefore, in this sense, this embodiment has a great practical advantage.

(Embodiment 3) FIG. 5 is an optical system and signal processing block diagram of another embodiment of the error signal generating device according to the present invention. In FIG. 5, 11 is a beam splitter, 12 is a reflecting mirror, 13 and 14 are photoelectric converters, and 15 is a subtracter that obtains a difference signal between the outputs of the photoelectric converters 13 and 14. The interaction length between the laser beam transmitted through the beam splitter 11 and the wavelength conversion element 2 is lb, and the interaction length between the laser beam reflected by the beam splitter 11 and the wavelength conversion element 2 is la. Therefore, the outputs of the photoelectric converters 13 and 14 are as shown in FIG.
This corresponds to graph 2 shown by the solid line and graph 1 shown by the broken line. A subtracter 15 calculates the difference signal between the outputs of the photoelectric converters 13 and 14.
By using , it is possible to obtain a control error signal which is a zero-crossing signal similar to that obtained in the first embodiment.

The controller 6 can stabilize the wavelength of the laser light by inputting this error signal and controlling the injection current of the semiconductor laser 1 so that the output of the subtracter 15 becomes zero.

As a specific example, a trapezoidal cross section with a base of 2 mm, a top of 1.96 mm, and a height of 20 mm is used as the wavelength conversion element 2, and the interval between the two beams is 1 mm.
When set to , the difference between la and lb is about 2 μm. If this is applied to FIG. 1, a specific error signal can be obtained.

Similar to the second embodiment, this embodiment also has a great practical advantage because it is not necessary to apply wavelength modulation to the laser beam when generating an error signal. In the above-described embodiments, wavelength stabilization of a semiconductor laser was described, but the present invention can be similarly applied to wavelength stabilization of gas lasers, solid-state lasers, liquid lasers, and the like.

[0033]

As described above, by using the present invention, an error signal for stabilizing the wavelength of laser light can be generated for any wavelength. Therefore, the wavelength of laser light can be stabilized for any wavelength with a simple configuration.

[Brief explanation of drawings]

FIG. 1 is a graph of (Equation 1) in which the power of light 2ω is plotted on the vertical axis and the angular frequency ω is plotted on the horizontal axis.

FIG. 2 is a graph of (Equation 1) in which the power of light 2ω is plotted on the vertical axis and the interaction length l is plotted on the horizontal axis.

FIG. 3 is an optical system and signal processing block diagram of an embodiment of the error signal generating device according to the present invention.

FIG. 4 is an optical system and signal processing block diagram of another embodiment of the error signal generating device according to the present invention.

FIG. 5 is an optical system and signal processing block diagram of another embodiment of the error signal generating device according to the present invention.

FIG. 6 is a block diagram of an optical system and signal processing for wavelength stabilization of a conventional semiconductor laser.

FIG. 7a shows an absorption line of rubidium (one of the Rb-D2 lines (wavelength 780.0 nm)) used for wavelength stabilization of a conventional semiconductor laser, and FIG. 7b shows an absorption line of rubidium used for wavelength stabilization of a conventional semiconductor laser. This is a differential signal obtained by differentiating the amount of transmitted light with respect to wavelength.

[Explanation of symbols]

1 Semiconductor laser 2 Wavelength conversion element 3 Photoelectric converter 4 Oscillator 5 Lock-in amplifier 6 Controller 7 Adder 8 Filter 9 that transmits light with angular frequency 2ω and blocks light with angular frequency ω X-axis stage 10 Oscillator 11 Beam splitter 12 Reflection mirrors 13, 14 Photoelectric converter 15 Subtractor 20 Rb cell

Claims (3)

[Claims] 1. An error signal generation device for stabilizing the wavelength of laser light, comprising means for modulating the wavelength of laser light, a wavelength conversion element, and a photoelectric converter for detecting the wavelength-converted light beam. 2. Error in wavelength stabilization of laser light, comprising a wavelength conversion element, means for modulating the interaction length between the laser light and the wavelength conversion element, and a photoelectric converter for detecting the wavelength-converted light beam. Signal generator. 3. A beam splitter that splits a laser beam into two light beams, a wavelength conversion element, two photoelectric converters that detect each of the wavelength-converted light beams, and a difference between the outputs of the photoelectric converters. An error signal generation device for stabilizing the wavelength of laser light, comprising a subtracter for obtaining a signal, and arranged so that the interaction lengths between the two light beams split by the beam splitter and the wavelength conversion element are different from each other. .